The RFeB-based magnet was found by Sagawa et. al. in 1982, and has an advantage that many magnetic properties such as residual magnetic flux density are higher than that of permanent magnets in the related art. Accordingly, the RFeB-based magnet has been used in various products such as a drive motor of a hybrid car and an electric car, a motor for electrically-assisted bicycles, an industrial motor, a voice coil motor of a hard disk drive and the like, a high-performance speaker, a headphone, and a permanent magnet-type magnetic resonance diagnostic device.
Early RFeB-based magnets have a defect that among various magnetic properties, a coercive force Hcj is relatively low. The coercive force represents a force that resists magnetization inversion when a magnetic field (opposing magnetic field) in a direction opposite to a direction of the magnetization is applied to a magnet Generally, as a temperature is raised, an effect of thermal fluctuation increases, and thus a coercive force decreases. Therefore, even when the opposing magnetic field has intensity to a degree in which spontaneous magnetization is not inverted at room temperature, magnetization is apt to be inverted at a certain temperature or higher. In addition, when comparing the same magnets to each other, a magnet having a high coercive force at room temperature also tends to have a high coercive force at a high temperature. This temperature dependency is also true of an RFeB-based magnet. An early RFeB-based magnet may not be used as a magnet such as a magnet for a drive motor of a vehicle which is used in an environment in which a use temperature increases to approximately 200° C.
It has been found that the coercive force is improved by making at least one element selected from the group consisting of Dy, Tb and Ho (hereinafter, at least one element selected from the group consisting of Dy, Tb, and Ho is referred to as a “heavy rare earth element RH”) be present inside the RFeB-based magnet. It is considered that the heavy rare earth element RH has an effect of hindering the above-described magnetization inversion. According to this, an RFeB-based magnet, in which inversion of magnetization does not occur even in a case of being used in a high-temperature environment similar to a drive motor of a vehicle, is obtained.
On the other hand, in the case where an amount of the RH increases, residual magnetic flux density Br decreases, and thus there is a problem that the maximum energy product (BH)max also decreases. In addition, the RH is a rare resource and is expensive, and a production area is localized, and thus it is not preferable to increase the amount of RH.
As a first method of improving the coercive force Hcj while suppressing a decrease in the residual magnetic flux density Br, a grain boundary diffusion method is known (For example, refer to Patent Document 1). In the grain boundary diffusion method, a powder which contains the RH as an elementary substance, a compound, or an alloy, or the like is attached to a surface of the RFeB-based magnet, and the RFeB-based magnet is heated, thereby penetrating the RH to the inside of the magnet through a grain boundary of the RFeB-based magnet. According to this, atoms of the RH diffuse only to the vicinity of the surface of respective crystal grains. There is a characteristic that magnetization inversion in a magnet occurs at first in the vicinity of a grain boundary of crystal grains, and diffuses to the inside of the crystal grains from the vicinity of the grain boundary, and thus when the atoms of the RH are allowed to diffuse in the vicinity of the surface of the crystal grains by the grain boundary diffusion method, it is possible to prevent the magnetization inversion at the grain boundary, and according to this, it is possible to prevent the magnetization inversion of the entirety of the magnet. In addition, it is possible to suppress an amount of the RH in the entirety of the magnet, and it is possible to prevent a decrease in the residual magnetic flux density Br.
In addition, the RFeB-based magnet is largely classified into (i) a sintered magnet obtained by sintering a raw material alloy powder containing a main phase grain as a main component, (ii) a bonded magnet obtained by tightening raw material alloy powders with a binding agent (binder composed of an organic material such as a polymer and an elastomer) and by molding the tightened powders, and (iii) a hot-plastic worked magnet obtained by performing a hot press working and hot plastic working with respect to a raw material alloy powder (refer to Non-Patent Document 1). Among these magnets, the grain boundary diffusion treatment may be performed in (i) sintered magnet and (iii) hot-plastic worked magnet in which the binder of the organic material is not used and thus heating during the grain boundary diffusion treatment can be performed.
As a second method of improving the coercive force Hcj while suppressing a decrease in the residual magnetic flux density Br, there is a method of locally increasing a content rate of the RH at a portion in which an adverse effect due to the decrease in the coercive force H becomes particularly significant in the entirety of the magnet. For example, with regard to an RFeB-based magnet that is used in a rotor of a motor, Patent Document 2 discloses a configuration in which the content rate of the RH at a portion in the vicinity of a stator in which a magnetic field is relatively strong in the motor is made to be higher than that of other portions. According to this, it is possible to suppress a decrease in the residual magnetic flux density Br in the entirety of the magnet while reliably increasing the coercive force at a portion in which it is necessary to improve the coercive force. In Patent Document 2, to locally increase the content rate of the RH as described above, a method for producing an RFeB-based magnet serving as a rotor of a motor by bonding a plurality of RFeB-based magnets manufactured by using raw materials in which content rates of the RH are different from each other is used.
[Patent Document 1] JP-A-2006-303433
[Patent Document 2] JP-A-2012-191211
[Patent Document 3] JP-A-2006-019521
[Non-Patent Document 1]“Development of Dy-omitted Nd—Fe—B-based hot worked magnet by using a rapidly quenched powder as a raw material” written by HIOKI Keiko and HATTORI Atsushi, Sokeizai, Vol. 52, No. 8, pages 19 to 24, General Incorporation Foundation of Sokeizai Center, published in August, 2011